1. Field of the Invention
The present invention relates to an image forming apparatus having an image carrier such as a photosensitive drum.
2. Description of Related Art
In an image forming apparatus such as a copying machines or a printer, an electrostatic latent image is formed on a photosensitive drum, developed with a toner, and then the developed toner image is transferred on a sheet. In such image forming apparatus, the sheet in the apparatus may be conveyed using an endless belt (refer to, for example, Japanese Unexamined Patent Publication No. 2003-29552).
The belt is extended by a driving roller and a driven roller and rotationally moved by the driving of the driving roller. The belt is held between the photosensitive drum and a transfer roller and a position in which the belt is held is defined as a transfer position. At the time of image formation, the sheet is conveyed by the belt to the transfer position and the toner image on the photosensitive drum is transferred due to transfer bias of the transfer roller.
The photosensitive drum is in contact with the belt at the transfer position and a foreign matter (hard material such as sand) may enter at the transfer position. At this time, when there is a difference in speed between the photosensitive drum and the belt at the transfer position, the foreign matter rubs against the photosensitive drum, causing a scratch on the surface of the photosensitive drum. Especially at the time of start of an image forming operation, since the photosensitive drum and the belt are each transited from the stop state to the driving state, the speed difference between the photosensitive drum and the belt is easy to occur, thereby causing a scratch on the photosensitive drum. If the photosensitive drum is scratched, the scratched region cannot be charged, resulting in deterioration in image quality.
The belt is hung about the driving roller and the driven roller with a slight slack in the stop state. As a result, it requires a time period from when the driving roller starts driving until the belt is applied a sufficient tension to move at a constant speed. In other words, the time during which the belt reaches the constant speed for the image forming operation is needed.